To provide a circuit breaker and a distribution board, which can accurately measure the current even if an unusual condition occurs in a power system.

A branch breaker 1 is a circuit breaker comprising: a casing 1a having a primary side terminal part 10 connected to a power system and a secondary side terminal part 11 connected to a load and housing a switch part 12 by inserting it between the terminal parts 10 and 11; a switch mechanism section 14 for opening/closing the switch part 12 in response to operation of an operating section 13 provided in the casing 1a, being exposed; a releasing section 15 for releasing the switch part 12 when an unusual condition occurs; a current detecting part 17 for detecting the current flowing in an electrical passage between the terminal parts 10 and 11; and a computing section 18 having a current computing section 180 for computing a current value of the electrical passage based on an output of detection by the current detecting section 17, and a communicating section 181 for transmitting a result of computing of the current computing section 180 to the inspecting device 6. The computing section 18 receives power from a power source 66 electrically separated from the power system.